Electrosurgical power control

ABSTRACT

A method and apparatus include determining a value of a parameter associated with operation of an electrosurgical probe having a particular probe design, and determining whether the value of the parameter is within a range of values that has been predetermined for the particular probe design to indicate that the probe is treating tissue in a desired manner. Power is delivered to the probe according to an algorithm based upon a determination that the value of the parameter is outside the range of values The algorithm delivers power in a pulsed profile including portions of low power and portions of high power. In one embodiment, the tissue treatment is ablation, the parameter is impedance, and the method limits tissue necrosis to less than 200 microns. In another embodiment, the tissue treatment is shrinkage, the parameter is temperature, and the method limits power delivery when the probe is not shrinking tissue.

TECHNICAL FIELD

This application relates to electrosurgical power control.

BACKGROUND

Radiofrequency (RF) energy is delivered to a surgical instrument, suchas a probe, to treat diseased tissue, such as by ablating, shrinking,cutting, or coagulating the tissue. For example, RF energy is used toablate fibrillations and smooth the surface of articular cartilage thatsuffers from chondromalacia and osteoarthritis. RF energy also is usedto shrink collagen tissue in a joint. The use of RF energy can producecollateral damage in the form of undesired cell death or the excessremoval of healthy tissue. For example, in the case of articularcartilage, RF energy can cause the death of chondrocytes, the cellsresponsible for maintaining cartilage viability and growth, which cannotbe regenerated after death.

SUMMARY

In various described embodiments, power delivered to a probe iscontrolled to treat tissue in a desired manner (e.g., ablating, cutting,shrinking, or coagulating) while also limiting the power delivered tothe probe when the probe is not treating tissue in the desired manner(e.g., when the probe is positioned too far from the tissue to achievethe desired tissue effect). Limiting the power may limit undesiredsurgical outcomes, e.g., by limiting the extent of undesired collateralcell death. In several such embodiments, that value of a parameterassociated with operation of the probe (e.g., impedance or temperature)is determined and compared to a range of values for that parameter thathas been predetermined, for that particular probe design, to indicatethat the probe is treating tissue in the desired manner. For example, animpedance of over 1000 ohms can indicate that a probe is ablating tissueand, for example, a temperature between approximately 75° C. and 85° C.can indicate that a probe is shrinking tissue. If the parameter isoutside the range of values for that probe, the probe enters a pulsedpower mode to limit the power delivered. By limiting the power deliveredwhen the probe is not treating tissue in the desired manner, theundesired surgical outcome is limited. The range of values for theparameter can vary based on the probe design and the desired manner oftreatment.

According to a general aspect, a method includes determining a value ofa parameter. The parameter is associated with operation of anelectrosurgical probe having a particular probe design. The methodincludes determining whether the value of the parameter is within arange of values that has been predetermined for the particular probedesign to indicate that the probe is treating tissue in a desiredmanner. The method further includes delivering power to the probeaccording to an algorithm based upon a determination that the value ofthe parameter is outside the range of values. The algorithm includes apulsed profile including portions of low power and portions of highpower.

Embodiments may include one or more of the following features.

The parameter may include an impedance. The range of the impedance maybe between approximately 50 ohms and approximately 4000 ohms. Thedesired manner of tissue treatment may include ablation. The low powerin the pulsed profile may be limited to a duration that causessubstantially no noticeable delay in initiating ablation. The durationof the pulsed low power may be between approximately 50 milliseconds andapproximately 500 milliseconds. The low power in the pulsed profile mayinclude a power setting between approximately 0 watts and approximately50 watts. The high power in the pulsed profile may have a durationsubstantially equal to a minimum length sufficient to initiate ablation.The high power may include a power setting between approximately 40watts and approximately 300 watts.

The parameter may include a temperature. The range of the temperaturemay be between approximately 65° C. and approximately and approximately90° C. The desired manner of tissue treatment may include shrinkage. Thelow power in the pulsed profile may be limited to a duration that limitsa delay in initiating shrinkage. The low power in the pulsed profile mayinclude a power setting between approximately 0 watts and approximately20 watts. The high power in the pulsed profile may have a duration thatis greater than a time needed for the temperature to reach a lower limitof the range when the probe is shrinking tissue and that is less than atime needed for the temperature to reach a lower limit of the range whenthe probe is not shrinking tissue. The high power may include a powersetting between approximately 10 watts and approximately 300 watts.

Power may be delivered to the probe according to a second algorithmdifferent from the algorithm to treat tissue in the desired manner basedupon a determination that the value of the parameter is within the rangeof values. The method may switch from the algorithm to the secondalgorithm upon a determination that the parameter is greater than alower limit of the range of values by a margin. The method may switchfrom the second algorithm to the algorithm upon a determination that theparameter is less than a lower limit of the range of values by a margin.

A specific number of values of the parameter may be determined. Thespecific number may be greater than one. For less than all of thespecific number of values, it may be determined whether the values ofthe parameter are within the predetermined range of values.

According to another general aspect, an apparatus includes one or morecomputer readable media having instructions stored thereon andconfigured to result in at least the following. A value of a parameterassociated with operation of an electrosurgical probe having aparticular probe design is determined. Whether the value of theparameter is within a range of values that has been predetermined forthe particular probe design to indicate that the probe is treatingtissue in a desired manner is determined. Power is delivered to theprobe according to an algorithm based upon a determination that thevalue of the parameter is outside the range of values. The algorithm hasa pulsed profile including portions of low power and portions of highpower.

According to another general aspect, a method includes making one ormore determinations that an impedance value encountered by anelectrosurgical probe is less than a threshold value. The method alsoincludes limiting tissue necrosis to less than 200 microns by deliveringpower to the electrosurgical probe, in response to at least one of theone or more determinations that the impedance is less than the thresholdvalue, according to an algorithm having a pulsed profile of low and highpower.

Embodiments may include one or more of the following features.

For example, the method may include limiting the low power in the pulsedprofile to a duration that causes substantially no noticeable delay ininitiating the ablative mode. The duration of the pulsed low power maybe between approximately 50 milliseconds and approximately 500milliseconds, e.g., approximately 201.5 milliseconds. Limiting tissuenecrosis may include providing the high power in the pulsed profile witha duration substantially equal to a minimum length sufficient toinitiate delivery of power in an ablative mode. The duration of thepulsed high power may be between approximately 10 milliseconds andapproximately 100 milliseconds, e.g., approximately 19.5 milliseconds.Limiting tissue necrosis may also include providing the low power in thepulsed profile between approximately 0 watts and approximately 50 watts,e.g., approximately 10 watts. The threshold value may be betweenapproximately 50 ohms and approximately 4000 ohms, e.g., approximately1000 ohms.

The method may include checking the impedance value betweenapproximately every 1 millisecond and approximately every 10milliseconds, e.g., approximately every 6.5 milliseconds, in order todetermine if the impedance value is less than the threshold value. Thehigh power may include a power setting between approximately 40 wattsand approximately 300 watts, e.g., approximately 60 watts, with aduration between approximately 10 milliseconds and approximately 100milliseconds, e.g., approximately 19.5 milliseconds. The low power mayinclude a power setting of between approximately 0 watts andapproximately 50 watts, e.g., approximately 10 watts, with a durationbetween approximately 50 milliseconds and approximately 500milliseconds, e.g., approximately 201.5 milliseconds.

The method may include making one or more determinations that theimpedance value encountered by the electrosurgical probe exceeds thethreshold value, and delivering power according to a second algorithm inresponse to at least one of the one or more determinations that theimpedance exceeds the threshold value. The second algorithm may includedelivering a substantially constant power. The method may includechanging from the second algorithm to the pulsed profile in less thanapproximately 10 milliseconds, e.g., approximately 1 millisecond, afterdetermining that the impedance is less than the threshold value. Themethod may include changing from the pulsed profile to the secondalgorithm upon a determination that the impedance exceeds a valuebetween approximately 50 ohms and approximately 4000 ohms, e.g.,approximately 1100 ohms. The method may include changing from the secondalgorithm to the pulsed profile upon a determination that the impedanceis less than a value between approximately 50 ohms and approximately4000 ohms, e.g., approximately 1000 ohms.

According to another general aspect, an electrosurgical probe isconfigured to deliver power to tissue. A control module is configured tomake one or more determinations that an impedance value encountered bythe electrosurgical probe is less than a threshold value. The controlmodule is further configured to limit tissue necrosis to less than 200microns by delivering power to the electrosurgical probe, in response toat least one of the one or more determinations that the impedance isless than the threshold value, according to an algorithm having a pulsedprofile of low and high power.

Embodiments may include one or more of the following features. The probemay include the control module. A generator may be configured to providepower to the probe, and the generator may include the control module.

According to another general aspect, a method includes determiningwhether a probe is delivering power in an ablative mode or anon-ablative mode. Power is delivered according to a first algorithmwhen in the ablative mode, and power is delivered according to a secondalgorithm different from the first algorithm when in the non-ablativemode. The second algorithm includes a pulsed profile including portionsof low power and portions of high power. The portions of high power areof sufficient durations to initiate the ablative mode and the durationsare less than 250 milliseconds.

According to another general aspect, a method includes determining arate at which a temperature of a probe approaches a range oftemperatures that has been predetermined to enable shrinkage of tissue.The method also includes determining whether the rate is within a rangeof rates that has been predetermined to indicate that a distance fromthe probe to tissue is small enough to enable shrinkage of tissue at atemperature in the range of temperatures. The method includes deliveringpower to the probe according to an algorithm based upon a determinationthat the rate is outside the range of rates, the algorithm comprising apulsed profile including portions of low power and portions of highpower. In an embodiment, the range of rates has been predetermined basedon the probe design.

Advantages may include more consistent surgical outcomes despitevariations in surgical technique, and improved safety duringelectrosurgery.

One or more of the general aspects may be embodied in, e.g., a method oran apparatus. The apparatus may include a component configured toperform various operations, functions, or instructions.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features will be apparent fromthe description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is an illustration of a system including a generator and aprobe.

FIG. 1B is a flow chart showing a general power control method used tocontrol power delivery from the generator to the probe of FIG. 1A.

FIG. 2 is a schematic illustration of an embodiment of the probe of FIG.1A positioned to apply energy to tissue in an ablative mode.

FIG. 3 is a schematic illustration of the probe of FIG. 2 positioned toapply energy to tissue in a non-ablative mode.

FIG. 4 is a flow chart showing an embodiment of the power control methodof FIG. 1B used for ablation of tissue.

FIG. 5 is a graphical representation of power delivered to a probe bythe generator according to the power control method of FIG. 4.

FIG. 6 is a flow chart showing an ablation algorithm of the powercontrol method of FIG. 4.

FIG. 7 is a flow chart showing a non-ablation algorithm of the powercontrol method of FIG. 4.

FIG. 8 is a graphical representation of the power output according to anembodiment of the non-ablation algorithm described in FIG. 7.

FIG. 9 is a flow chart showing another embodiment of the power controlmethod of FIG. 1B used for shrinkage of tissue.

FIG. 10 is a flow chart showing a non-shrinkage algorithm of the powercontrol method of FIG. 9.

FIG. 11 is a flow chart showing a shrinkage algorithm of the powercontrol method of FIG. 9.

DETAILED DESCRIPTION

Referring to FIG. 1A, an electrosurgical probe 200 is coupled to an RFgenerator 100 to apply RF energy to treat tissue in a desired manner,e.g., to ablate, shrink, cut, or coagulate tissue. Generator 100 can be,for example, a Vulcan® generator sold by Smith & Nephew, Inc., ofMemphis, Tenn. (catalog no. 7210812). Generator 100 includes a display115, control buttons 120, status indicators 130, a receptacle 140 for agrounding pad (not shown), and a receptacle 145 that receives probe 200via a cable plug 160, a cable 155, and a cable plug 162. Probe 200includes a handle 201, a shaft 204 extending from handle 201, and anelectrode 206 coupled to a distal end 202 of shaft 204 for applyingenergy to tissue. The instructions for generator controls, as describedin greater detail below, can be implemented in hardware or software, canbe built into generator 100 or probe 200, and can be stored on one ormore computer readable media, such as one or more memory cards.

Delivering power according to a high generator power setting may berequired to treat tissue in a desired manner. However, delivering poweraccording to the same high generator power setting when the probe is notoperating in the desired manner can contribute to adverse surgicaloutcomes, such as collateral cell death. As a result, when treatingtissue with RF energy, it is desirable to keep the probe operating inthe desired manner (e.g., ablating or shrinking tissue) as much aspossible, and to avoid delivering high power (e.g., power sufficient toablate or shrink tissue) when the probe is not operating in the desiredmanner. Further, when a surgeon moves a probe, e.g., from one portion oftissue to another, the probe's distance to tissue can vary, which cancause the probe to switch between delivering power in the desired mannerand delivering power in another manner. To facilitate more consistentsurgical outcomes, it is desirable to allow the probe to quickly switchbetween delivering power in the desired manner and delivering power inanother manner when the distance to tissue is varied. In many cases,whether the probe is operating in the desired manner can be determined,on a probe-by-probe basis, by determining a parameter associated withprobe operation (e.g., impedance or temperature) and comparing thatparameter to a range of values that has been predetermined, for thatparticular probe design, to indicate that the probe is operating in thedesired manner.

Referring to FIG. 1B, generator 100 controls the power delivered toprobe 200 according to an algorithm 10 when probe 200 is used to treattissue. In typical embodiments, generator 100 initially receives oraccesses the type and configuration of the probe being used in thetissue treatment (12), the desired tissue treatment to be performed bythe probe (14), a range of values for a parameter (e.g., impedance ortemperature) that has been predetermined to indicate that the particularprobe is treating tissue in the desired manner (16), and a desired powersetting for the probe (18). Additionally, one or more of these inputsmay be received implicitly, e.g., by user selection of an algorithm fora particular probe type and configuration, or use of a generator thatonly supports one probe type and configuration. These inputs also may bereceived by being built into an algorithm (e.g., range of parametervalues or power settings). These inputs can be received, e.g., viamanual user input or by automated storage and retrieval of information,according to known methods. These inputs are specific to the probedesign and can vary based on the probe being used.

Generator 100 then delivers power to probe 200 according to a tissuetreatment algorithm that is designed to deliver power in a manner thatallows probe 200 to treat tissue in the desired manner, e.g., bydelivering power according to a constant power setting or according to adynamic control algorithm such as a proportional-integral-derivative(PID) control algorithm (20). In an embodiment, substantially constantpower can be delivered according to a dynamic control algorithm thatadjusts voltage and/or current to attempt to maintain power Whiledelivering power according to the tissue treatment algorithm, algorithm10 periodically determines the value of the parameter of interest (22)and compares the determined value of the parameter with the range ofvalues that has been predetermined to indicate that the probe istreating tissue in the desired manner (24). In an embodiment, algorithm10 determines a specific number of values of the parameter, the specificnumber being greater than one, and determines, for less than all of thespecific number of values, whether the values of the parameter arewithin the predetermined range of values.

If the determined value of the parameter is within the range of values(“yes” branch from 24), probe 200 is determined to be treating tissue inthe desired manner, and generator 100 continues to deliver poweraccording to the tissue treatment algorithm (20). If the determinedvalue of the parameter is outside the range of values (“no” branch from24), probe 200 is determined not to be treating tissue in the desiredmanner, and generator 100 switches to delivering power according to anon-treatment algorithm that pulses the power setting between a highpower value and a low power value (26). In either case, algorithm 10continues to periodically determine the value of the parameter ofinterest (22) and to compare the determined value of the parameter withthe range of values that indicates that probe 200 is treating tissue inthe desired manner (24). If the determined value of the parameter iswithin the range of values (“yes” branch from 24), generator 100delivers power according to the tissue treatment algorithm (20). If thedetermined value of the parameter is outside the range of values (“no”branch from 22), generator 100 delivers power according to thenon-treatment algorithm (26).

Referring to FIGS. 2 and 3, in one embodiment, RF power is deliveredfrom generator 100 to electrode 206 to smooth cartilage tissue 300 byablation as probe 200 is moved across the surface of tissue 300. Probe200 is monopolar, such that energy passes from electrode 206, throughthe tissue 300 and surrounding saline, to a return electrode pad (notshown) attached to receptacle 140 (FIG. 1) and placed elsewhere on thepatient. Probe 200 can be, for example, a Glider probe (catalog no.7210438) or a Sculptor probe (catalog no. 7210697) sold by Smith &Nephew, Inc. In an alternative embodiment, the probe could be bipolarsuch that a return electrode is located on the probe (not shown).

When probe 200 is placed sufficiently close to or in contact with thetissue surface 300, e.g., approximately 0 mm to approximately 5 mm fromthe tissue surface 300, as illustrated by the position of probe 200 inFIG. 2 (not to scale), power can be delivered to probe 200 sufficient toablate the tissue. When probe 200 is ablating tissue, probe 200 is in an“ablative mode.” In the ablative mode, an electrical arc or plasmadischarge is formed between probe 200 and the tissue and saline suchthat a light appears to be given off from the probe-tissue interface,and the electrical arc ablates the cells of the tissue.

When probe 200 is moved further away from the tissue surface 300, e.g.,at least approximately 0.5 mm to at least approximately 5 mm, asillustrated by the position of probe 200 in FIG. 3 (not to scale), andprobe 200 is no longer able to ablate the tissue, the probe is in a“non-ablative mode.” In the non-ablative mode, energy from probe 200heats the fluid surrounding the tissue, which can heat the surroundingtissue to a temperature that causes cell death, an undesired surgicaloutcome. For example, chondrocyte cells in articular cartilage cellstend to die when raised to a temperature that is greater thanapproximately 55° C. This can occur, for example, when the surgeon movesprobe 200 away from the tissue to reposition probe 200 on the tissuesurface while the probe is delivering RF energy.

The ablative mode and the non-ablative mode are characterized by aparameter that has been predetermined, for the design of probe 200, toindicate that probe 200 is operating in, e.g., the ablative mode or thenon-ablative mode. In an embodiment, operation in the ablative mode isindicated by an impedance encountered by the probe. For example, animpedance that falls within a range that is greater than a thresholdvalue, e.g., greater than a value between approximately 50 ohms andapproximately 4000 ohms, indicates that the probe is determined to beoperating in the ablative mode. When the impedance is outside thisrange, e.g., less than the threshold value, the probe is operating inthe non-ablative mode. In another embodiment, a high temperature atprobe 200, e.g., greater than approximately 100° C., can indicate thatthe probe is operating in the ablative mode.

The impedance is higher when the probe is operating in the ablativemode, at least in part, due to the high impedance of the arc/discharge,and to the higher impedance of tissue, as compared to surrounding fluid.Lower impedance indicates operation in the non-ablative mode, at leastin part, due to the lower impedance of the surrounding fluid. Theimpedance threshold that indicates ablation can be determinedempirically, e.g., by delivering a constant output power to the probeand determining the impedance when the probe is operating in thenon-ablative mode (e.g., by operating the probe in saline) and theimpedance when the probe is operating in the ablative mode (e.g., byusing the probe to ablate a tissue sample). Operation in the ablativemode is indicated, e.g., by light that appears to be given off from theprobe-tissue interface. After several values of impedance in thenon-ablative and ablative modes have been determined, a threshold valuefor the impedance can be computed. A typical threshold indicatingoperation in the ablative mode may be, e.g., the lowest value ofimpedance that always (or with a certain degree of confidence) indicatesthat the ablation is occurring. In another embodiment, the thresholdthat indicates operation in the ablative mode may be selected, as, e.g.,the lowest value of impedance that indicates ablation and that providesa margin of a predetermined size such that the threshold can bedecreased by a predetermined size and still indicate ablation.

The threshold value for impedance that indicates probe operation in theablative mode can vary based on generator design, probe design, and thesurgical environment. For example, impedance can vary based on thesurface area of the electrode and materials used in construction of theprobe. For example, an electrode having a larger surface area may have alower impedance threshold than a probe with a smaller electrode surfacearea. Insulating materials of the probe that have poorer dielectricproperties can allow more current leakage and reduce the impedancevalues. Current carrying components of the probe with a lowerconductivity can contribute to higher impedance values. In oneembodiment, for the Glider probe, the threshold value of the impedancehas been predetermined to be approximately 1000 ohms. In anotherembodiment, for the Sculptor probe, which has a larger electrode surfacearea than the Glider probe, the threshold value of the impedance hasbeen predetermined to be approximately 700 ohms.

Referring to FIG. 4, a first embodiment of a power control algorithm 400uses ablation and non-ablation algorithms to control the power output toprobe 200. Generator 100 receives an input of the desired power levelfrom an operator of probe 200 (402). Generator 100 initially deliverspower to probe 200 according to an ablation algorithm that is designedto deliver a power level sufficient to allow probe 200 to operate in theablative mode, as described in more detail below with respect to FIG. 6(404). Algorithm 400 then periodically determines, while continuing todeliver power according to the ablation algorithm, whether probe 200 isdelivering power in the ablative mode or the non-ablative mode (406). Ifprobe 200 is operating in the ablative mode (“yes” branch from 406),generator 100 continues to deliver power according to the ablationalgorithm (404).

If the power control algorithm 400 determines that probe 200 isoperating in the non-ablative mode (“no” branch from 406), generator 100switches to delivering power according to a non-ablation algorithmdesigned to limit the amount of power delivered to the tissue, asdescribed in more detail below with respect to FIG. 7 (408). Algorithm400 continues to periodically determine, while continuing to deliverpower according to the non-ablation algorithm, whether probe 200 isdelivering power in the ablative mode or the non-ablative mode (406) andquickly switches back to the ablation algorithm (404) upon adetermination that probe 200 is operating in the ablative mode.

In certain instances, generator 100 may be controlled by the ablationalgorithm even though probe 200 is operating in the non-ablative mode,e.g., if there is a lag in algorithm 400 determining that probe 200 hasswitched to the non-ablative mode. Conversely, in certain instances,generator 100 may be controlled by the non-ablation algorithm eventhough probe 200 is operating in the ablative mode, e.g., if there is alag in algorithm 400 determining that probe 200 has switched to theablative mode. Although the power control algorithm is depicted with theablation algorithm being the initial setting, in another embodiment thenon-ablation algorithm is the initial setting.

Referring to FIG. 5, power control algorithm 400 causes generator 100 toproduce an exemplary power profile 500 having portions 510, 520, and 530that correspond to the ablation algorithm, the non-ablation algorithm,and the ablation algorithm, respectively. The power levels generated bythe ablation and non-ablation algorithms and depicted in FIG. 5 arepower settings for generator 100. The output to probe 200 is, e.g., analternating current waveform in the form of a sinusoidal RF waveform,that can average to a value at or below the power setting. For example,the power can be equal to the time average of the product of the averagecurrent, the average voltage, and the cosine of the phase shift betweenthe current and voltage. Also, due to the impedance encountered by probe200, e.g., from saline or tissue, the average power output fromgenerator 100 and probe 200 can be less than the power setting. Forexample, for a power setting of 60 Watts, if probe 200 is in saline(low-impedance) the average power output can be, e.g., approximately 50Watts, and if probe 200 is being applied to tissue (high-impedance), theaverage power output can be, e.g., approximately 10 Watts. This can bedue, for example, to voltage or current limits in generator 100.However, other embodiments can deliver power that is close orsubstantially equal to the set power if the generator has, for example,higher voltage or current limits. The power settings described below arethe power settings of the generator, unless otherwise indicated.

Initially, as illustrated by portion 510, probe 200 is operating in theablative mode, and the ablation algorithm (404) causes generator 100 toproduce a power level in accordance with, e.g., a substantially constantpower setting (P-SET) on the generator (402). In another embodiment, theablation algorithm causes generator 100 to produce power according to adynamic control algorithm, such as a PID control algorithm. At line 515,probe 200 is determined to be operating in the non-ablative mode (406).The power control algorithm 400 then switches to the non-ablationalgorithm (408) that causes generator 100 to produce a pulsed powerlevel, as illustrated by portion 520. The pulsed power alternatesbetween high power pulses (P-HI) having a duration of T-HI and low powerpulses (P-LO) having a duration of T-LO, to limit the amount of energydelivered to tissue, and thus limit the amount of collateral cell death.

At some point in portion 520, for example, during one of the high powerpulses as indicated by line 525, probe 200 is determined to be operatingin the ablative mode (406). The power control algorithm then switchesback to the ablation algorithm (404) that causes generator 100 todeliver power in accordance with the operator determined power setting(P-SET). The high power pulses of the non-ablation algorithm aresufficient in power and in duration to allow the probe to return to theablative mode when, for example, the probe is positioned close totissue. The power control algorithm 400 can be further configured toprovide a quick return, e.g., within approximately 0 to 6.5milliseconds, to the ablation algorithm when probe 200 returns to theablative mode.

Referring to FIG. 6, an ablation algorithm 600 controls power deliverywhen probe 200 is determined to be operating in the ablative mode.According to the ablation algorithm 600, a power setting (P-SET) is usedto control the voltage and current to probe 200 to cause probe 200 toablate the tissue (602). The power setting (P-SET) can be constant orvariable and can be substantially equal to the setting inputted by anoperator or can vary from the operator's setting. For example, P-SET maybe between approximately 40 watts and approximately 300 Watts. Threeexemplary values for P-SET (constant values of 60 W, 65 W, and 70 W) foruse with the Glider probe are set forth below in Table 1. An exemplaryvalue of P-SET for use with the Sculptor probe is a substantiallyconstant value of approximately 150 Watts.

While continuing to deliver power sufficient to ablate tissue, theimpedance encountered by probe 200 is periodically determined, e.g.,between approximately every 1 millisecond and approximately every 10milliseconds, by determining the voltage and the current across probe200, to determine whether probe 200 is operating in the ablative mode(604). Each cycle of the algorithm that checks the impedance encounteredby probe 200 is an “impedance check cycle.” For the examples for theGlider probe set forth below in Table 1 and for the Sculptor probe, theduration of each impedance check cycle is approximately 6.5milliseconds. The determined impedance is compared to a range of values,e.g., greater than a value between approximately 1000 ohms for theGlider probe and greater than approximately 700 ohms for the Sculptorprobe, that have been determined to indicate when probe 200 is operatingin the ablative and non-ablative modes (606). If the impedance is withinthis range of values (“no” branch from 606), then ablative powercontinues to be delivered to probe 200 (602). If the impedance dropsbelow this range of values (“yes” branch from 606), it is determinedthat probe 200 is no longer ablating tissue, and generator 100 switchesto a non-ablation algorithm (608).

The lower limit value of the range of impedance values used forcomparison when switching from the ablation algorithm to thenon-ablation algorithm (606) may be lower, by a margin, than the lowerlimit of the range that has been predetermined, as discussed above, toindicate that probe 200 is operating in the ablative mode. This lowervalue still indicates that the probe is operating in the ablative mode,but provides a hysteresis buffer before switching from the ablationalgorithm to the non-ablation algorithm. For example, for the Gliderprobe, the lower limit of the impedance range that indicates operationin the ablative mode is approximately 1000 ohms, but the lower limitused for comparison when determining whether to switch from thenon-ablation algorithm to the ablation algorithm is approximately 900ohms. In another embodiment, for the Sculptor probe, the lower limit ofthe impedance range that indicates operation in the ablative mode isapproximately 700 ohms, but the lower limit used for comparison whendetermining whether to switch from the non-ablation algorithm to theablation algorithm is approximately 650 ohms.

The change from the ablation algorithm to the non-ablation algorithmoccurs substantially immediately, e.g., within approximately 0 to 6.5milliseconds, after the probe enters the non-ablative mode. Thesubstantially immediate change is achieved because the delay in changingto the non-ablation algorithm is largely determined by the delay indetermining that probe 200 is not ablating tissue, which is largelydetermined by the delay in measuring impedance. However, the impedanceis measured frequently, e.g., approximately every 1 millisecond toapproximately every 10 milliseconds, such as approximately every 6.5milliseconds. The impedance can be determined at other regular orirregular intervals. Further, other embodiments can impose an additionaldelay in changing to the non-ablation algorithm in order, for example,to maintain a probe's ability to enter the ablative mode while a probeoperator moves a probe from one tissue location to another and the probemomentarily moves to the non-ablative mode.

Referring to FIG. 7, a non-ablation algorithm 700 controls the pulsedpower delivered to probe 200 when probe 200 is determined to beoperating in the non-ablative mode (608). To begin the high powerportion of the pulsed power, a cycle counter is set to zero (702) andincremented by one (704). A high power setting (P-HI) is used to controlvoltage and/or current applied to probe 200 (706). The value of P-HI issufficiently high so that probe 200 can operate in the ablative mode,e.g., approximately 40 watts to approximately 300 Watts. Exemplaryvalues for P-HI for the three power settings of P-SET for the Gliderprobe are 60 Watts, 65 Watts, and 70 Watts, as set forth below in Table1, and an exemplary value for P-HI for the power setting P-SET of 150Watts for the Sculptor probe is approximately 150 Watts. While theexemplary values of P-HI are equal to the exemplary values of P-SET, thevalues of P-HI also could be greater than or less than the values ofP-SET. That is, the high-power setting in the non-ablative mode'snon-ablation algorithm 700 need not be the same as the power setting inthe ablative mode's ablation algorithm 600, but can be higher or lower.Note that, if probe 200 is not ablating tissue, the actual powerdelivered to probe 200 can be close to the power setting of P-HI, e.g.,50 Watts delivered for a 60 Watt power setting. If probe 200 is ablatingtissue, the actual power delivered can be lower than the setting ofP-HI, due to the higher impedance, e.g., approximately 10 Wattsdelivered for a 60 Watt setting.

During the high power pulse, the impedance is determined, e.g.,approximately every 1 millisecond to approximately every 10milliseconds, by determining, e.g., the voltage and current across probe200 (708). The determined impedance is compared to the predeterminedimpedance range that indicates ablation of tissue, e.g., greater than avalue between approximately 50 ohms and approximately 4000 ohms (710). Avalue within this range indicates that probe 200 has switched from thenon-ablative mode to the ablative mode. If the impedance is within thisrange (“yes” branch from 710), then generator 100 switches back to theablation algorithm 600 described above (712).

When switching from the non-ablation algorithm to the ablationalgorithm, the lower limit of the range used for comparison (710) may behigher, by a margin, than the lower limit of the impedance used forcomparison when switching from the ablation algorithm to thenon-ablation algorithm. The margin still indicates that the probe isoperating in the ablative mode and provides a hysteresis buffer beforeswitching from the non-ablation algorithm to the ablation algorithm. Forexample, for the Glider probe, the lower limit of the impedance isapproximately 1000 ohms, but the lower limit value used for comparisonwhen determining whether to switch from the ablation algorithm to thenon-ablation algorithm is approximately 1100 ohms. In anotherembodiment, for the Sculptor probe, the lower limit of the impedance isapproximately 700 ohms, but the lower limit value used for comparisonwhen determining whether to switch from the ablation algorithm to thenon-ablation algorithm is approximately 750 ohms.

If the impedance is not within the range (“no” branch from 710),non-ablation algorithm 700 checks whether the cycle counter has reachedthe maximum number of cycles for the high power portion of the algorithm(714). T-HI is equal to the maximum number of cycles multiplied by thelength of each impedance determining cycle. T-HI is selected to be atleast the minimum time necessary to allow probe 200 to enter theablative mode, e.g., at least approximately 10 milliseconds, whilelimiting the amount of energy being delivered if probe 200 is operatingin the non-ablative mode, e.g., approximately 100 milliseconds or less.Exemplary values of T-HI for the Glider probe (which are multiples of anexemplary impedance determining cycle of 6.5 milliseconds) are set forthbelow in Table 1. An exemplary value for T-HI for the Sculptor probe isapproximately 30 milliseconds. If the number of cycles has not reachedthe maximum (“no” branch from 714), then generator 100 increments thecycle counter (704) and continues to use the P-HI setting to controlpower output to probe 200 (706). If the number of cycles has reached themaximum (“yes” branch from 714), generator 100 switches to the low powerportion of non-ablation algorithm 700.

To begin the low power portion of the pulse, the cycle counter is resetto zero (716) and is incremented by one (718). A low power setting(P-LO) is used to control the amount of voltage and/or current appliedto probe 200 (720). The value of P-LO is chosen so as to limit theamount of energy applied to tissue, and thus the amount of cell death,e.g., approximately 50 Watts or less, while still allowing probe 200 toeasily switch back to P-HI with little or no ramp-up time, e.g.,approximately 0 Watts or more. Exemplary values for P-LO areapproximately 10 Watts for the Glider probe, as shown below in Table 1,and approximately 10 Watts for the Sculptor probe. Note that, becauseprobe 200 is not ablating tissue, the actual power delivered to probe200 typically will be close to the actual power setting of P-LO, e.g.,approximately 3-5 Watts delivered for a 10 Watt power setting.

During the low power pulse, the impedance is determined, e.g.,approximately every 1 millisecond to approximately 10 milliseconds, bydetermining, e.g., the voltage and current across probe 200 (722). Thedetermined impedance is compared to the range of values that indicatesthe probe is ablating tissue, with or without an additional margin, asdiscussed above (724). If the impedance is within this range (“yes”branch from 724), probe 200 is switched back to the ablation algorithm(712). However, it can be unlikely that the impedance will be withinthis range during the low power pulse because power being delivered toprobe 200 is typically too low for probe 200 to operate in the ablativemode. Thus, even if probe 200 is moved close enough to the tissue toablate the tissue, probe 200 typically will remain in the non-ablativemode until the high power pulse (P-HI) is delivered to probe 200.

If the impedance is below the lower limit of this range (“no” branchfrom 724), non-ablation algorithm 700 checks whether the cycle counterhas reached the maximum number of impedance check cycles for the lowpower portion of non-ablation algorithm 700 (726). T-LO is equal to themaximum number of impedance check cycles multiplied by the length ofeach cycle, and is selected to be, e.g., as long as possible so as tomaximize the amount of time that P-LO is applied to the tissue to reducecell death, e.g., between approximately 50 milliseconds andapproximately 500 milliseconds. However, T-LO can also be limited sothat non-ablation algorithm 700 switches to T-HI with sufficientfrequency that there is substantially no noticeable delay, as perceivedby the probe operator, in the probe switching to the ablative mode,e.g., approximately 0 to 6.5 milliseconds. Exemplary values of T-LO forthe Glider probe (which are multiples of an exemplary impedancedetermining cycle of 6.5 milliseconds), as well as the average delay inreturning to the ablative mode, are set forth below in Table 1. Theaverage delay is equal to the average of T-HI and T-LO. An exemplaryvalue of T-LO for the Sculptor probe is approximately 200 milliseconds.

Table 1 shows exemplary values for P-SET, P-HI, T-HI, P-LO, T-LO, andthe average delay in change to ablative mode for generator settings of60 Watts, 65 Watts, and 70 Watts for the design of the Glider probe. Theaverage delay in change to the ablative mode are examples of delay timesthat cause no noticeable delay to the operator in switching from thenon-ablative mode to the ablative mode. In contrast, embodiments forwhich T-LO is excessively long can cause a perceivable delay to a probeoperator before the probe is allowed to change to ablative mode.Further, embodiments for which T-HI is too long can unnecessarily heatsaline, whereas the values of T-HI in Table 1 are substantially theminimum sufficient to initiate delivery of power in the ablative mode.

TABLE 1 Exemplary Powers and Durations for Glider Probe P-SET P-LO Avg.Delay in Change to (W) P-HI (W) T-HI (msec) (W) T-LO (msec) AblativeMode (msec) 60 60 19.5 (3 cycles) 10 201.5 (21 cycles) 120 msec 65 6526.0 (4 cycles) 10 201.5 (21 cycles) 127 msec 70 70 32.5 (5 cycles) 10201.5 (21 cycles) 133 msec

If the number of cycles has not reached the maximum (“no” branch from726), then generator 100 increments the cycle counter (718) andcontinues to use P-LO to control power output to probe 200 (720). If thenumber of cycles has reached the maximum (“yes” branch from 726),generator 100 switches to the high power portion of the non-ablationalgorithm 700 (702-714). Generator 100 is able to increase to P-HIsubstantially instantaneously, e.g., within approximately 0 to 1millisecond, at the end of T-LO, although in other embodiments there canbe a more gradual ramp-up to P-HI.

Referring to FIG. 8, the non-ablation algorithm 700 can result in asubstantial reduction in overall power, and thus limit collateral celldeath, during operation of probe 200 and, in particular, during thenon-ablative mode. FIG. 8 shows the power output over time from a Gliderprobe operated according to one full cycle of non-ablation algorithm 700(the darker colored plot indicated by A) and with a constant powersetting for the same period of time (the lighter colored plot indicatedby B) in saline solution with generator 100 power setting set to 60Watts. The average power output according to the non-ablation algorithm700 is approximately 6.7 Watts, while the average power output accordingto the constant power setting is approximately 51.7 Watts. Thus, thehigh and low power pulses of the non-ablation algorithm result in areduction of power of approximately 87% as compared to a constant powersetting.

As shown below in Table 2, experimental use of algorithm 400 onarticular cartilage samples from a healthy bovine patella placed in roomtemperature saline showed average tissue necrosis being limited to lessthan 200 μm (average mean cell death of 108 μm and average maximum celldeath of 167 μm) when power was applied according to algorithm 400 witha Glider probe positioned too far from the tissue to operate in theablative mode. The articular cartilage was treated for 30 seconds at apower setting of 60 Watts while the power output and impedance wererecorded. The experiment was performed three times with three differentGlider probes. The average power output from the generator wasapproximately 7.3 Watts, an approximately 88% reduction from the 60 Wattsetting. Each impedance measurement, and the average impedance, was lessthan 1000 ohms, indicating that the probe was operating in thenon-ablative mode. The error for each value in Table 2 is plus or minusone standard deviation.

TABLE 2 Mean cell Max cell Avg. Avg. Probe death (μm) death (μm) Power(W) Impedance (Ω) 1 302 470 7 863 2 23 31 7 156 3 0 0 8 156 Avg. 108 1677.3 392

The values set forth above are specific to the design of the Gliderprobe. Other types of probes, such as the Sculptor probe, and othertypes of treatments may need to be calibrated with different ranges ofvalues of the parameter being measured and different values of P-SET,P-HI, T-HI, P-LO and/or T-LO.

Referring to FIGS. 9-11, in another embodiment, an algorithm 900 is usedto control power delivery from generator 100 to a probe, based on thetemperature of the probe electrode, in order to shrink tissue. The probecan be, for example, a TAC-S probe (catalog no. 7209633) or a mini-TAC-Sprobe (catalog no. 7209632), sold by Smith & Nephew, Inc. Tissueshrinkage occurs, for example, when the temperature of the tissue isbetween approximately 55° C. and approximately 100° C. When thetemperature of the tissue is below or above this range, tissue does notshrink, and collateral tissue damage or death can occur. When the probeis not-shrinking tissue it is operating in the “non-shrinkage mode.”

In an embodiment, shrinkage of tissue is indicated by a temperature ofthe probe electrode, e.g., between approximately 65° C. andapproximately 90° C. The temperature range for the probe that indicatestissue shrinkage can vary based upon the generator power (P-SET) andtemperature (Temp-SET) settings. The range of electrode temperature thatindicates tissue shrinkage can be determined empirically for each probe,for example, by applying the probe to tissue samples, setting thegenerator to operate according to a dynamic control algorithm (e.g.,according to a proportional-integral-derivative temperature controlalgorithm), and observing the temperature at which tissue shrinkageoccurs.

The electrode temperature can vary based upon the probe design andconfiguration, and the power and temperature settings on the generator.For example, a probe having a larger surface area may have a narrowerrange of probe temperatures that achieve the tissue temperature requiredfor shrinkage, because a larger mass electrode has a larger thermalmass, causing slower temperature changes in the probe electrode. In oneembodiment, tissue shrinkage is indicated by a temperature of the TAC-Sprobe of approximately 75° C. to 85° C., with generator settings ofapproximately 20 Watts and approximately 75° C. In another embodiment,tissue shrinkage is indicated by a temperature of the mini-TAC-S probeof approximately 75° C. to 900° C., with generator settings ofapproximately 20 Watts and approximately 75° C. The temperature rangesthat indicate tissue shrinkage may have lower limits that are less thanthe lower limits in these ranges and tipper limits that are higher thanthe upper limits in these ranges.

The shrinkage mode typically occurs when the probe is placedsufficiently close to or in contact with the tissue surface, e.g.,approximately 0 mm to approximately 5 mm from the tissue surface. Whenthe probe is moved further away from the tissue surface, e.g., at leastapproximately 0.5 mm to at least approximately 5 mm, the probe typicallyoperates in the “non-shrinkage mode.” When the probe is close enough tothe tissue to shrink the tissue and the temperature is within the rangeto shrink the tissue, the probe is operating in the “shrinkage mode.”When the probe is further away from the tissue and/or the temperature isnot within the range to shrink tissue, the probe is operating in the“non-shrinkage mode.”

When the probe is close enough to tissue to operate in the shrinkagemode, the temperature of the electrode tends to rapidly reach thedesired range of electrode temperatures that indicates tissue shrinkage,e.g., within approximately 1.5 seconds for the TAC-S probe and withinapproximately 1.0 seconds for the mini-TAC-S probe. When the probe isoperating further from tissue, e.g., in the non-shrinkage mode, thetemperature of the electrode tends to reach the desired temperaturerange that indicates shrinkage more slowly, if at all, e.g.,approximately 10 seconds for the TAC-S probe and greater than 10seconds, or not at all, for the mini-TAC-S probe. The time it takes theprobe electrode to reach the shrinkage mode temperature range can beobserved empirically by operating the probe at a constant power settingin saline and when applied to tissue, and determining the amount of timeeach takes to reach the shrinkage mode temperature range.

It is desirable to limit the amount of energy delivered to the probewhen operating away from tissue and to determine when the probe is closeenough to tissue to operate in the shrinkage mode. The temperature rangethat indicates operation in the shrinkage mode and the time that ittakes the electrode to reach this temperature range in the shrinkagemode and in the non-shrinkage mode can be used to indicate whether theprobe is operating in the shrinkage mode or the non-shrinkage mode,according to power control algorithm 900, described in greater detailbelow.

Referring to FIG. 9, power control algorithm 900 uses shrinkage andnon-shrinkage algorithms to control the power to the probe. Generator100 receives input of the desired power level from an operator of theprobe (P-SET) (902). For example, as shown below in Table 3, the TAC-Sprobe uses a P-SET of approximately 20 Watts and the mini-TAC-S probeuses a P-SET of approximately 20 Watts. Generator 100 initially deliverspower to the probe according to a non-shrinkage algorithm designed tolimit the amount of power delivered to the tissue by delivering a pulsedpower to the probe, as described in more detail below with respect toFIG. 10 (904). Algorithm 900 then periodically determines, whilecontinuing to deliver power according to the non-shrinkage algorithm,whether the probe is delivering power in the shrinkage mode or thenon-shrinkage mode (906). If the probe is operating in the non-shrinkagemode (“no” branch from 906), generator 100 continues to deliver poweraccording to the non-shrinkage algorithm (904).

If the power control algorithm 900 determines that the probe isoperating in the non-shrinkage mode (“yes” branch from 906), generator100 switches to a shrinkage algorithm that is designed to deliver powerin a manner that allows the probe to continue to operate in theshrinkage mode, as described in more detail below with respect to FIG.11 (908). Algorithm 400 continues to periodically determine, whilecontinuing to deliver power according to the shrinkage algorithm,whether the probe is operating in the shrinkage mode or thenon-shrinkage mode (906) and quickly switches back to the non-shrinkagealgorithm (904) upon a determination that the probe is operating in thenon-shrinkage mode.

In certain instances, generator 100 may be controlled by the shrinkagealgorithm even though the probe is operating in the non-shrinkage mode,e.g., if there is a lag in algorithm 900 determining that the probe hasswitched to the non-shrinkage mode. Conversely, in certain instances,generator 100 may be controlled by the non-shrinkage algorithm eventhough the probe is operating in the shrinkage mode, e.g., if there is alag in algorithm 900 determining that the probe has switched to theshrinkage mode. Although the power control algorithm is depicted withthe non-shrinkage algorithm being the initial setting, in anotherembodiment the shrinkage algorithm is the initial setting.

Referring to FIG. 10, a non-shrinkage algorithm 1000 controls pulsedpower having portions of high power and portions of low power deliveredto the probe when the probe is determined to be operating in thenon-shrinkage mode (904). To begin the high power portion of the pulsedpower, a cycle counter is set to zero (1002) and incremented by one(1004). A high power setting (P-HI) is used to control voltage and/orcurrent applied to the probe so as to deliver a high power pulse (1006).The value of P-HI is sufficiently high so that the probe can reach thetemperature range that indicates that the probe is close enough totissue to operate in the shrinkage mode within the duration of T-HI. Forexample, as shown below in Table 3, the value of P-HI is approximately40 Watts for the TAC-S probe and approximately 20 Watts for themini-TAC-S probe. The values of P-HI also may be greater than, lessthan, or equal to the power setting on the generator.

During the high power pulse, the temperature of the probe electrode isdetermined, e.g., approximately every 1 millisecond to approximatelyevery 100 milliseconds, by a thermocouple located within the probe(1008). Each cycle of determining temperature is a “temperaturedetermining cycle.” The determined temperature is compared to a range oftemperatures that indicates shrinkage of tissue, e.g., betweenapproximately 75° C. and approximately 85° C. for the TAC-S probe andbetween approximately 75° C. and approximately 90° C. for the mini-TAC-Sprobe (1010).

If the temperature is within this range (“yes” branch from 1010), thengenerator 100 switches to a shrinkage algorithm 1100 described below(1012). In an embodiment, the lower limit of the range used forcomparison to the temperature of the probe is greater than the lowerlimit of the range that indicates shrinkage of tissue, by a margin,e.g., approximately 2° C., to provide a hysteresis buffer beforeswitching from the non-shrinkage algorithm to the shrinkage algorithm.For example, generator 100 switches from the non-shrinkage algorithm1000 to the shrinkage algorithm 1100 when the temperature is betweenapproximately 77° C. and approximately 85° C. for the TAC-S probe, andbetween approximately 77° C. and approximately 90° C. for the mini-TAC-Sprobe.

If the temperature is not within the range (“no” branch from 1010),non-shrinkage algorithm 1000 checks whether the cycle counter hasreached the maximum number of cycles for the high power portion of thealgorithm (1014). If the number of cycles has not reached the maximum(“no” branch from 1014), then generator 100 increments the cycle counter(1004) and continues to use the P-HI setting to control power output tothe probe (1006). If the number of cycles has reached the maximum (“yes”branch from 1014), generator 100 switches to the low power portion ofnon-shrinkage algorithm 1000.

The duration of the high power pulse (T-HI) is equal to the maximumnumber of cycles multiplied by the length of each temperaturedetermining cycle. T-HI is greater than the time period required for theprobe to reach the desired temperature range when shrinking tissue, andless than the time period required for the probe to reach the desiredtemperature range when not shrinking tissue, which is determined asdescribed above. For example, for the TAC-S probe, T-HI is approximately2 seconds, which is greater than the time required for the TAC-S probeto reach the desired temperature range when shrinking tissue (e.g.,approximately 0.5 seconds to 1.5 seconds) but less than the timerequired to reach the temperature range when not shrinking tissue (e.g.,greater than approximately 10 seconds). Similarly, for the mini-TAC-Sprobe T-HI is approximately 1.5 seconds, which is greater than the timerequired for the mini-TAC-S probe to reach the desired temperature rangewhen shrinking tissue (e.g., approximately 0.5 seconds to 1 second) butless than the time required to reach the temperature range when notshrinking tissue (e.g., greater than approximately 10 seconds). Thus,algorithm 1000 switches to the low power pulse if the probe has notreached the temperature that indicates shrinkage within the time periodof T-HI, and switches to the shrinkage algorithm if the probe hasreached the temperature that indicates shrinkage within the time periodof T-HI. In this way, algorithm 1000 is able to make a determination asto whether the probe is shrinking tissue.

To begin the low power portion of the pulse, the cycle counter is resetto zero (1016) and is incremented by one (1018). A low power setting(P-LO) is used to control the amount of voltage and/or current appliedto the probe to deliver a low power pulse (1020). The value of P-LO ischosen so as to limit the amount of energy applied to tissue, and thusthe amount of cell death, e.g., approximately 5 Watts or less for theTAC-S and the mini-TAC-S probes, while still allowing the probe toeasily switch back to P-HI with little or no ramp-up time. During thelow power pulse, the temperature of the electrode is determined, e.g.,approximately every 1 millisecond to approximately 10 milliseconds(1022). The determined temperature is compared to the range of valuesthat indicates the probe is shrinking tissue, as discussed above (1024).If the temperature is within this range (“yes” branch from 1024), theprobe is switched to the shrinkage algorithm (1012). However, it can beunlikely that the temperature will be within this range during the lowpower pulse because power being delivered to the probe is typically toolow for the probe to achieve the temperature range. Thus, even if theprobe is moved close to the tissue, the probe typically will remain inthe non-shrinkage mode until the high power pulse (P-HI) is delivered tothe probe.

If the temperature is below the lower limit of this range (“no” branchfrom 1024), non-shrinkage algorithm 1000 checks whether the cyclecounter has reached the maximum number of temperature check cycles forthe low power portion of non-shrinkage algorithm 1000 (1026). T-LO isequal to the maximum number of impedance check cycles multiplied by thelength of each cycle, and is selected to be, e.g., as long as possibleso as to maximize the amount of time that P-LO is applied to the tissueto reduce cell death, e.g., between approximately 1 second andapproximately 3 seconds. For example, T-LO is approximately 2 secondsfor the TAC-S and the mini-TAC-S probes.

If the number of cycles has not reached the maximum (“no” branch from1026), then generator 100 increments the cycle counter (1018) andcontinues to use P-LO to control power output to the probe (1020). Ifthe number of cycles has reached the maximum (“yes” branch from 1026),generator 100 switches to the high power portion of the non-shrinkagealgorithm 1000 (1002-1014). Generator 100 is able to increase to P-HIsubstantially instantaneously, e.g., within approximately 0 to 1millisecond, at the end of T-LO, although in other embodiments there canbe a more gradual ramp-up to P-HI.

Referring to FIG. 11, a shrinkage algorithm 1100 controls power deliverywhen the probe is determined to be operating in the shrinkage mode.According to the shrinkage algorithm 1100, power is delivered to theprobe according to, e.g., a dynamic control algorithm, based on thepower setting on the generator (P-SET) (1102). In one embodiment, thedynamic control algorithm is a known proportional-integral-derivative(PID) temperature control algorithm that attempts to maintain the probeelectrode temperature within the desired temperature range, e.g.,between approximately 75° C. and approximately 85° C. for the TAC-Sprobe and between approximately 75° C. and approximately 90° C. for themini-TAC-S probe. In other embodiments, the dynamic control algorithmcan be a constant or variable power output and can be substantiallyequal to the setting inputted by an operator or can vary from theoperator's setting.

While continuing to deliver power according to the dynamic controlalgorithm, the temperature of the probe electrode is also periodicallydetermined by algorithm 1100, e.g., between approximately every 1millisecond and approximately every 10 milliseconds, to determinewhether the probe is operating in the shrinkage mode (1104). Forexample, the temperature is determined approximately every 6.5milliseconds. The determined temperature is compared to the desiredrange of temperature values, as described above, that have beendetermined to indicate when the probe is operating in the shrinkage andnon-shrinkage modes (1106). If the temperature is within this range ofvalues (“yes” branch from 1106), then power continues to be delivered tothe probe according to the temperature control algorithm (1102). If thetemperature is outside this range of values (“no” branch from 1106), itis determined that the probe is no longer shrinking tissue, andgenerator 100 switches to a non-shrinkage algorithm (1108).

In an embodiment, the lower limit of the range used for comparison tothe temperature of the probe is less than the lower limit of the rangethat indicates shrinkage of tissue, by a margin, e.g., of approximately2° C. These lower limits still indicate that the probe is shrinkingtissue and provide a hysteresis buffer before switching from theshrinkage algorithm to the non-shrinkage algorithm. For example,generator 100 switches to the shrinkage algorithm 1100 when thetemperature is between approximately 73° C. and approximately 85° C. forthe TAC-S probe, and between approximately 73° C. and approximately 90°C. for the mini-TAC-S probe.

If the temperature is below this range of values, the change from theshrinkage algorithm to the non-shrinkage algorithm occurs substantiallyimmediately, e.g., within approximately 0 to 1 millisecond, after thealgorithm determines that the temperature is below the range of values.The substantially immediate change is achieved because the delay inchanging to the non-shrinkage algorithm is largely determined by thedelay in determining that the probe is not shrinking tissue, which islargely determined by the delay in measuring temperature. However, thetemperature is measured frequently, e.g., approximately every 1millisecond to approximately every 10 milliseconds, such asapproximately every 6.5 milliseconds. The temperature can be determinedat other regular or irregular intervals. Further, other embodiments canimpose an additional delay in changing to the non-shrinkage algorithm inorder, for example, to maintain a probe's ability to enter the shrinkagemode while a probe operator moves a probe from one tissue location toanother and the probe momentarily moves to the non-shrinkage mode. Ifthe temperature is above this range of values, generator 100 can shutoff power delivery to the probe completely for a period of time to allowthe tissue to cool before switching to the non-shrinkage algorithm. Inother embodiments, generator 100 can require the user to manuallyrestart power delivery to the probe after shutting off power, canperform other algorithms, and can have a hysteresis buffer at the upperlimit of the temperature range.

Table 3 shows exemplary values for the temperature setting on thegenerator (Temp-SET), the temperature range that indicates shrinkage(Temp Range), the power setting on the generator (P-SET), the powersetting for the high power portion of the pulsed power (P-HI), theduration of the high power pulse (T-HI), the power setting for the lowpower portion of the pulsed power (P-LO), and the duration of the lowpower portion of the pulse (T-LO) for embodiments of the algorithm 900using the TAC-S probe and the mini-TAC-S probe.

TABLE 3 Exemplary Values for TAC-S Probe and Mini-TAC-S Probe Mini-TAC-STAC-S Probe Probe Temp-SET 75° C. 75° C. Temp Range 75° C. to 85° C. 75°C. to 90° C. P-SET 20 Watts 20 Watts P-HI 40 Watts 20 Watts P-LO 5 Watts5 Watts T-HI 2 seconds 1.5 seconds T-LO 2 seconds 2 seconds

In another embodiment for delivering energy to a probe to shrink tissue,the rate at which the probe's temperature approaches the range oftemperatures that indicates shrinkage of tissue, as described above withrespect to algorithm 900, is used to determine whether the probe isclose enough to tissue to be shrinking the tissue. While deliveringpower according to a constant power setting of the generator, the rateat which the temperature of the probe approaches the range oftemperatures that indicate shrinkage is determined, e.g., by computing aderivative of the probe temperature with respect to time.

The rate of temperature increase is compared to a range of rates. Therange of rates has been predetermined to indicate whether the probe isclose enough to tissue to shrink the tissue at, for example, atemperature within the range of temperatures. A slower rate indicatesthat the probe is positioned too far from the tissue to shrink thetissue. A faster rate indicates that the probe is positioned closeenough to tissue to shrink tissue. If the rate is outside of thepredetermined range of rates, the generator delivers power according tothe pulsed mode of the non-shrinkage algorithm 1000, as described above.If the rate is within the range of rates, the generator delivers poweraccording to the shrinkage algorithm 1100 described above.

The range of rates can vary based upon several factors, including probedesign and generator settings. In one embodiment, for a TAC-S probeoperating at a power setting of 20 Watts and a temperature setting of75° C., the range of rates that indicates that the probe is positionedclose enough to tissue to initiate or sustain shrinkage is aboveapproximately 20° C./second. In another embodiment, for a mini-TAC-Sprobe operating at a power setting of 10 Watts and a temperature settingof approximately 75° C., the range of rates that indicates that theprobe is positioned close enough to tissue to initiate or sustainshrinkage is above approximately 40° C./second. In certain embodiments,the range of rates determined for one probe design can be used for adifferent probe design. In another embodiment, the rate can becontinuously monitored, and the generator can switch between theshrinkage algorithm and the non-shrinkage algorithm based on the rate oftemperature change.

In another embodiment, an algorithm analogous to algorithm 900 can beused to control power output to a probe, e.g., a TAC-S probe, to shrinktissue, based upon monitoring the impedance encountered by the probe.Initially, the probe delivers a power according to a dynamic controlalgorithm, such as a temperature PID dynamic control algorithm, to aprobe, such as the TAC-S probe, sufficient to shrink tissue. Thealgorithm then periodically determines the impedance encountered by theprobe and compares the impedance to a range of impedance values that hasbeen predetermined to indicate that the probe is shrinking tissue. Theimpedance encountered by the probe will increase as the tissue shrinks.If the impedance is within the range of impedance values that indicatesthe probe is shrinking tissue, the algorithm continues to deliver powerto the probe according to the dynamic control algorithm. If theimpedance is greater than the upper limit of the range, the algorithmcauses the generator to switch to a pulsed power setting. Duringdelivery of the pulsed power setting, the algorithm continues to monitorthe impedance encountered by the probe. If the impedance is stillgreater than the upper limit of the range, the algorithm continues todeliver the pulsed power to the probe. If the impedance decreases to bewithin the range that indicates shrinkage of tissue, the algorithmcauses the generator to switch back to the dynamic control algorithm.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made. The algorithm can beconfigured for use with other types of probes having a variety of shapesand sizes and in a variety of types of procedures. For example, thealgorithm can be used with the following probes manufactured by Smith &Nephew, Inc., in the following procedures: the Glider probe for use inthermal chondroplasty in the knee, hip, shoulder, or hand; the Sculptor,Ablator (catalog no. 7209654), or Saphyre (catalog no. 7209686) probefor use in subacromial decompression in the shoulder, meniscectomy inthe knee, or synovectomy in the shoulder, knee, wrist, ankle, hip,elbow, or hand; or a Ligament Chisel (catalog no. 7209649) for use intriangular fibrocartilage complex (TFCC) debridement or carpal tunnelrelease in the wrist. A single probe can be used for multiple treatmentmodes using different algorithms.

Parameters other than impedance and temperature, such as voltage,current, or power, can be monitored in order to determine whether theprobe is operating in the desired manner, or whether to switch betweenalgorithms. Two or more parameters can be determined together todetermine whether the probe is operating in the desired mode ofoperation. The desired range of values for the parameter can be closedended or open ended. The algorithm used when operating in the desiredmanner (e.g., the ablative mode) can have a constant or a variableoutput and can have its own dynamic control algorithm, such as a PIDcontroller. The pulsed power algorithm used when the probe is notoperating in the desired manner (e.g., the non-ablative mode) can bepulsed to more than two power levels or can have a waveform, such as asinusoidal wave. The parameter (e.g., impedance) can be monitored moreor less frequently. The values of P-SET, P-HI, T-HI, P-LO, and T-LO canbe higher or lower to achieve other objectives. The time for T-HI orT-LO can be counted in a different way such as by using a separatetiming circuit. In addition to or instead of changing power output, theoperator can be notified of, e.g., the non-ablative mode ornon-shrinkage mode by, for example, an audible alarm, a visual display,or complete shutdown of the system. The algorithms can be used tocontrol, e.g., power, current, and/or voltage delivered to the probe.

The impedance, voltage, current, temperature, or other parameters, canbe monitored for other reasons, such as to determine whether the probeis stationary for an undesired period of time. For example, if theimpedance rises above a threshold amount, indicating a stationary probe,the power to the probe can be shut off and the operator can be requiredto manually reinitiate ablation. In addition or in the alternative, theoperator can be warned by an audible alarm or a visual indication.

Implementations may include one or more devices configured to performone or more processes. A device may include, for example, discrete orintegrated hardware, firmware, and software. Implementations may beembodied in a processing device, such as, for example, a processor, amicroprocessor, an integrated circuit, or a programmable logic device.Implementations also may be embodied in a device, such as, for example,a volatile or non-volatile memory structure, such as, for example, ahard disk, a flash memory, a compact diskette, a random access memory,and a read-only memory. The memory structure may include one or morecomputer readable media having instructions for carrying out one or moreprocesses. The computer readable media may include, for example,magnetic or optically-readable media, and formatted electromagneticwaves encoding or transmitting instructions. Instructions may be, forexample, in hardware, firmware, software, or in an electromagnetic wave.A processing device may include, therefore, a device configured to carryout a process, or a device including computer readable media havinginstructions for carrying out a process.

These and other embodiments are within the scope of the followingclaims.

1. A method comprising: determining a value of a parameter, theparameter being associated with operation of an electrosurgical probehaving a particular probe design; determining whether the value of theparameter is within a range of values that has been predetermined forthe particular probe design to indicate that the probe is treatingtissue in a desired manner; and delivering power to the probe accordingto an algorithm based upon a determination that the value of theparameter is outside the range of values, the algorithm comprising apulsed profile including portions of a low power alternating waveformand portions of a high power alternating waveform, the high poweralternating waveform being sufficient to treat the tissue in the desiredmanner.
 2. The method of claim 1 wherein the parameter comprises animpedance.
 3. The method of claim 2 wherein the range of the impedanceis between approximately 50 ohms and approximately 4000 ohms.
 4. Themethod of claim 2 wherein the desired manner of tissue treatmentcomprises ablation.
 5. The method of claim 4 further comprising limitingthe low power alternating waveform in the pulsed profile to a durationthat causes substantially no noticeable delay in initiating ablation. 6.The method of claim 5 wherein the duration of the pulsed low poweralternating waveform is between approximately 50 milliseconds andapproximately 500 milliseconds.
 7. The method of claim 4 wherein the lowpower alternating waveform in the pulsed profile comprises a powersetting between approximately 0 watts and approximately 50 watts.
 8. Themethod of claim 4 further comprising providing the high poweralternating waveform in the pulsed profile with a duration substantiallyequal to a minimum length sufficient to initiate ablation.
 9. The methodof claim 4 wherein the high power alternating waveform comprises a powersetting between approximately 40 watts and approximately 300 watts. 10.The method of claim 1 wherein the parameter comprises a temperature. 11.The method of claim 10 wherein the range of the temperature is betweenapproximately 65° C. and approximately 90° C.
 12. The method of claim 11wherein the range of the temperature is between approximately 75° C. andapproximately 90° C.
 13. The method of claim 10 wherein the desiredmanner of tissue treatment comprises shrinkage.
 14. The method of claim13 further comprising limiting the low power alternating waveform in thepulsed profile to a duration that limits a delay in initiatingshrinkage.
 15. The method of claim 13 wherein the low power alternatingwaveform in the pulsed profile comprises a power setting betweenapproximately 0 watts and approximately 20 watts.
 16. The method ofclaim 13 further comprising providing the high power alternatingwaveform with a duration that is greater than a time needed for thetemperature to reach a lower limit of the range when the probe isshrinking tissue and that is less than a time needed for the temperatureto reach a lower limit of the range when the probe is not shrinkingtissue.
 17. The method of claim 13 wherein the high power alternatingwaveform comprises a power setting between approximately 10 watts andapproximately 300 watts.
 18. The method of claim 1 further comprisingdelivering power to the probe according to a second algorithm differentfrom the algorithm to treat tissue in the desired manner based upon adetermination that the value of the parameter is within the range ofvalues.
 19. The method of claim 18 further comprising switching from thealgorithm to the second algorithm upon a determination that theparameter is greater than a lower limit of the range of values by amargin.
 20. The method of claim 18 further comprising switching from thesecond algorithm to the algorithm upon a determination that theparameter is less than a lower limit of the range of values by a margin.21. The method of claim 1 further comprising: determining a specificnumber of values of the parameter, the specific number being greaterthan one; and determining, for less than all of the specific number ofvalues, whether the values of the parameter are within the predeterminedrange of values.
 22. The method of claim 18 wherein the second algorithmcauses constant delivery of the high power alternating waveform to theprobe based upon the determination that the value of the parameter iswithin the range of values.
 23. The method of claim 1, wherein the lowpower alternating waveform has a low average power output and the highpower alternating waveform has a high average power output, the highaverage power output being greater than the low average power output.24. An apparatus comprising one or more tangible computer readable mediahaving instructions stored thereon and configured to result in at leastthe following: determining a value of a parameter associated withoperation of an electrosurgical probe having a particular probe design;determining whether the value of the parameter is within a range ofvalues that has been predetermined for the particular probe design toindicate that the probe is treating tissue in a desired manner; anddelivering power to the probe according to an algorithm based upon adetermination that the value of the parameter is outside the range ofvalues, the algorithm comprising a pulsed profile including portions ofa low power alternating waveform and portions of a high poweralternating waveform, the high power alternating waveform beingsufficient to treat the tissue in the desired manner.
 25. The apparatusof claim 24, wherein the apparatus comprising one or more tangiblecomputer readable media having instructions stored thereon is furtherconfigured to result in: causing constant delivery of the high poweralternating waveform to the probe based upon a determination that thevalue of the parameter is within the range of values.
 26. The apparatusof claim 24, wherein the low power alternating waveform has a lowaverage power output and the high power alternating waveform has a highaverage power output, the high average power output being greater thanthe low average power output.
 27. The apparatus of claim 24, wherein thedesired manner of tissue treatment comprises ablation.
 28. The apparatusof claim 24, wherein the desired manner of tissue treatment comprisesshrinkage.
 29. A method comprising: determining that a value of aparameter associated with operation of an electrosurgical probe iswithin a predetermined range of values; delivering, in response to thedetermination that the value of the parameter is within thepredetermined range of values, a high power alternating waveform to theelectrosurgical probe; determining that a value of the parameterassociated with operation of the electrosurgical probe is outside thepredetermined range of values; and delivering, in response to thedetermination that the value of the parameter is outside thepredetermined range of values, a pulsed profile alternating between thehigh power alternating waveform and a low power alternating waveform tothe electrosurgical probe.